1. Field of the Invention
The present invention relates to a solid acid and a full cell using the same, and more particularly, to a sulfonated ionic conducting cross-linked copolymer having low methanol crossover and high ionic conductivity, and a fuel cell including the same.
2. Description of the Related Art
A fuel cell is an electrochemical device which directly transforms the chemical energy of hydrogen and oxygen which are contained in hydrocarbon materials such as methanol, ethanol, and natural gas into electric energy. The energy transformation process of a fuel cell is very efficient and environmentally-friendly.
Fuel cells can be classified into Phosphoric Acid Fuel Cells (PAFC), Molten Carbonate Fuel Cells (MCFC), Solid Oxide Full Cells (SOFC), Polymer Electrolyte Membrane Fuel Cells (PEMFC), and Alkaline Fuel Cells (AFC) according to the type of electrolyte used. All fuel cells operate on the same principle, but the type of fuel used, operating speed, the catalyst used and the electrolyte used are different. In particular, a PEMFC is capable of being used in small-sized stationary power generation equipment or a transportation system due to its high reaction speed, low operating temperature, high output density, rapid start-up, and sensitive response to output request variation.
The core part of a PEMFC is a Membrane and Electrode Assembly (MEA). An MEA generally includes a polymer electrolyte and 2 electrodes formed of a cathode and an anode, each of which is attached to the each side of the polymer electrolyte membrane. The polymer electrolyte membrane acts as a separator blocking direct contact between an oxidizing agent and a reducing agent, and electrically insulates the two electrodes while conducting protons. Accordingly, a good polymer electrolyte membrane has high proton conductivity, good electrical insulation, low reactant permeability, excellent thermal, chemical and mechanical stability under normal fuel cell conditions, and a reasonable price.
In order to meet these requirements, various types of polymer electrolyte membranes have been developed, and, in particular, a highly fluorinated polysulfonic acid membrane such as a NAFION™ membrane is a standard due to excellent durability and performance.
Moreover, in a Direct Methanol Fuel Cell (DMFC), an aqueous methanol solution is supplied as a fuel to the anode and a portion of non-reactive aqueous methanol solution is permeated to the polymer electrolyte membrane. The methanol solution that permeates to the polymer electrolyte membrane causes a swelling phenomenon in an electrolyte membrane, which makes the methanol solution diffuse to a cathode catalyst layer. Such a phenomenon is referred to as ‘methanol crossover.’ In this phenomenon, the direct oxidization of methanol occurs at the cathode where, otherwise, only electrochemical reduction of hydrogen ions and oxygen should occur. Therefore, electric potential can be degraded, thereby causing a significant decline in the performance of the fuel cell.
This issue is common in other fuel cells using a liquid fuel such as a polar organic fuel.
Since conventional polymer electrolyte membranes do not have both excellent ionic conductivity and low methanol crossover, improvements in polymer electrolyte membranes are required.